Not Applicable
The invention relates to biaxially textured metal substrates and articles made therefrom, and more particular to superconductor wires and tapes.
One important characteristic regarding applications of high-temperature superconducting wires and tapes is the amount of current the superconductor can carry. The current carrying capability of high-temperature superconductors is generally influenced by several factors, the most important ones being operating temperature, operating field, and the microstructure of the superconductor.
Operating temperature and field are external factors which can be generally controlled to the extent possible independently of the microstructure of the high temperature superconducting wire. The lower the operating temperature, the more current a superconducting wire can carry. Operating fields have a similar effect. The smaller the field, the more current a superconducting wire can carry. Unfortunately, many electric power powerline applications operate in high fields and at significant temperatures, the fields being up to 5 T and temperatures often being at least 50 K, or more. Thus, efforts to improve the current carrying capability of high temperature superconductor wires and tapes have focused on improving the microstructure of the superconductor layer.
The superconductor layer microstructure for both epitaxial thin film superconductors and superconducting tapes and wires is heavily influenced by the presence of grain boundaries. Epitaxial superconducting thin films on single crystal ceramic substrates such as SrTiO3 or LaAlO3 have no grain boundaries, are relatively small area films having areas generally up to several cm2, and possess the best high temperature superconductor properties known, having Jc values over 1 MA/cm2 at 77 K in self-field for YBCO. These superconducting thin films are currently used in device applications in the cellular industry, such as for filters in cellular base stations.
On the other hand, superconducting wires and tapes are used for applications such as power transmission lines, transformers, motors, generators, fault current limiters and magnets, and in medical applications as magnets for MRI and NMR. Each of these applications noted above generally require superconducting tape/wire lengths to be in the range of 100 m to 1 km, or more. Superconducting wires and tapes generally have critical current densities (Jc) which are generally 10 to 100 times less than epitaxial thin film high temperature superconductors. Thus, the microstructure of superconducting wires and tapes offers great potential for improvement as wires and tapes attempt to approach the superconducting properties provided by epitaxial thin film superconductor films.
Various manufacturing techniques exist to produce long length wire and tapes, the powder-in-tube (PIT) approach being the most commonly used [1-3]. References 1-3 use the PIT method to form uniaxially textured PIT tapes. In PIT, a superconducting powder is placed inside a hollowed tube, commonly a silver tube. The tube is generally evacuated, sealed, and then pulled (wire-drawn), stretched or otherwise deformed (rolled) to result in the desired shape of a tape or wire. After shaping, the tube is subjected to one or more heat treatments to result in some portion of the superconductor formed having superconducting properties. Similarly, using a xe2x80x9cthick filmxe2x80x9d process, a superconductor mixture is coated onto a metallic substrate having a shape of a tape or wire resulting from a deformation process, then the article is heat treated.
Powder-in-tube methods have proved somewhat successful for the Bi-(Pb)-Sr-Ca-Cu-O (BSCCO) family of superconductors. Typically the superconducting core, being the superconducting material closest to the Ag tube material, has a strong fiber texture with its c-axis aligned perpendicular to the tape surface. However, no in-plane texture is observed on a macroscopic or a local scale [4]. Recent electron backscatter measurements have revealed that such tapes have a grain boundary misorientation texture. Examination of over 227 spatially connected grain boundaries indicated that over 40% of the grain boundaries have misorientations angles which are less than 15 degrees. Percolative current paths consisting of such low angle boundaries can easily be traced in the material. It has been suggested that strongly linked current flow in these tapes is a direct consequence of the enhanced number of small angle grain boundaries [4]. These observations are consistent with measurements across single grain boundaries in various high temperature superconductor epitaxial thin films which indicate the strong dependence of critical current density (Jc) on misorientation angle of individual superconducting grains [5103].
Comparison of the Jc of typical PIT tapes to that of epitaxial films indicates that only approximately 1-5% of the tape cross-section is effectively carrying all of the current. This is consistent with the percolative nature of the current flow as suggested by the microstructure of the tapes [4]. Enhanced high temperature superconductor wires and tapes could be produced if the resulting texture could provide a larger percentage of the superconducting layer having primarily low angle grain boundaries.
A method to fabricate superconducting wire and tapes with properties similar to that obtained for epitaxial thin film superconductors on single crystal ceramic substrates has been proposed previously [6]. In this method, a polycrystalline, randomly oriented bar or rod of metal is thermomechanically processed to produce a biaxially textured substrate. Epitaxial oxide buffer layers and superconductors can then be grown on the biaxially textured metal substrate. This process can yield critical current densities higher than 1 MA/cm2 at 77K and self-field.
Such a technique is however is only generally applicable to the fabrication of thick film superconducting tapes. Furthermore, this technique can only be applied to those metals and alloys which preferentially form a single texture component, and produce biaxial texture upon rolling and recrystallization. Lastly, this technique can limit the attainable degree of biaxial texture that can be reproducibly produced even in metals/alloys which produce sharp biaxial texture upon rolling and annealing.
1. K. Sato, T. Hiakata, H. Mukai, M. Ueyama, T. Kato, T. Matsuda, M. Nagata, K. Iwata and T. Mitsui, IEEE Trans. Magn., 27 (1991) 1231.
2. K. Heine, J. Tenbrink and M. Thoner, Appl. Phys. Lett., 55 (1991) 2441.
3. R. Flukiger, B. Hensel, A. Jeremie, M. Decroux, H. Kupfer, W. Jahn, E. Seibt, W. Goldacker, Y. Yamada and J. Q. Xu, Supercond, Sci and Tech., 5 (1992) S61.
4. A. Goyal, E. D. Specht, D. M. Kroeger, T. A. Mason, D. J. Dingley, G. N. Riley and M. W. Rupich, Appl. Phys. Lett., 66 (1995) 2903-2905.
5. D. Dimos, P. Chaudhari, J. Mannhart and F. K. Legoues, Phys. Rev. Lett., 61 (1988) 219; D. Dimos, P. Chaudhari and J. Mannhart, Phts. Rev. B41 (1990) 4038.
6. U.S. Pat. No. 5,741,377 to Goyal et al.
A new method for forming improved biaxially textured metal and metal alloy substrates is presented. The method can be used to fabricate high critical current density superconducting tapes and powder-in-tube tapes, having lengths of up to 1 km, or more. The resulting superconducting wires and tapes obtainable using the invention have critical current densities (Jc) which more closely approach Jc values provided by epitaxial thin film high temperature superconductors, as compared to previous methods for forming superconducting wires and tapes.
A method for forming an electronically active biaxially textured article, preferable being a wire or tape, includes the steps of providing a substrate having a single crystal metal/alloy surface, deforming the substrate to form an elongated substrate surface having biaxial texture using a plurality of incremental deformations, and depositing an epitaxial electronically active layer on the biaxially textured surface.
The substrate can be made of any suitable metal or alloy. In some cases, the substrate is preferably a Ag or Ag alloy. A superconductor article having enhanced biaxial texture can be formed by this method. Rather than starting from a polycrystalline, randomly oriented bar or rod of a metal or alloy to produce a biaxial texture upon rolling, an embodiment of the invention starts with a single crystal metal/alloy rod, bar or tube, having a known crystallographic orientation. The deformation of the substrate of an appropriate crystallographic orientation can be performed by rolling without any intermediate anneals. In such cases, the crystallographic orientation selected is such that it has a stable crystal orientation with respect to rolling. This stable orientation can be maintained after heating the substrate at high temperatures.
The epitaxial electronically active layer can be a superconductor layer. Both the buffer layer and/or the superconducting layer can be deposited using a variety of processes, such as physical vapor deposition including pulsed laser ablation, sputtering, co-evaporation, pulsed electron beam evaporation, chemical vapor deposition (CVD) methods such as metallo-organic CVD or by solution routes such as dip coating, spray pyrolysis with metal organics or sol-gel solutions.
The superconductor layer may be an oxide superconductor. The oxide superconductor is preferably selected from REBa2Cu3O7, where RE is a rare earth element, (Bi,Pb)1Sr2Canxe2x88x921CunO2n+2, where n is an integer between 1 and 4, (Tl,Pb)1Ba2Canxe2x88x921CunO2n+3, where n is an integer between 1 and 4, and (Hg,Tl,Pb)1Ba2Canxe2x88x92CunO2n+2, where n is an integer between 1 and 4. It is noted that (Bi, Pb), (Tl,Pb) and (Hg, Tl, Pb) indicate doping of Pb, in (Tl, Pb) and (Bi, Pb) compounds and doping of Tl and Pb in (Hg, Tl, Pb) compounds, the doping being in any amount. Furthermore, Ca can be used as a dopant in RE for the REBa2Cu3O7 compound shown above.
The method can also include the step of annealing the substrate after substrate deformation to form a biaxially textured surface having a different crystallographic orientation from that of the starting single crystal material. In this embodiment, the crystallographic orientation selected is such that it is a stable crystal orientation with respect to rolling, but this orientation is changed to the desired orientation of the substrate after heating the substrate at high temperatures. Epitaxial buffer layers and/or electronic device layers such as a superconductor layer can then be deposited after this annealing step.
In an alternate embodiment, the method can also involve deformation of the substrate in a plurality of incremental deformations, each of which can be followed by annealing the substrate to regain any loss or spread of the original crystallographic orientation of the substrate. Successive thermomechanical processing can then used to produce a tape of the starting single crystal material having the desired dimensions.
An optional epitaxial buffer layer can be deposited on the biaxially textured substrate surface. The epitaxial buffer layer is preferably selected from SrTiO3, Nb-doped SrTiO3, LaMnO3, LaAlO3, La2ZrO3, YSZ, MgO, CeO2 and Y2O3.
In yet another embodiment of the invention, a method for forming textured alloy articles having biaxial texture is described. A single crystal metal substrate is provided which is deformed to form an elongated substrate surface having biaxial texture on the elongated substrate surface. A second metal, which is different from the first metal, is then diffused into the elongated substrate surface to form a biaxially textured alloy. The substrate may also be annealed to produce the biaxially texture substrate surface. A superconductor article having enhanced biaxial texture can be formed by this method.
Yet another embodiment of this invention is a method for forming an electronically active biaxially textured article, preferable being a wire or tape, by providing a single crystal metal/alloy tube, then filling the tube with superconducting or other electronically active prescursor powder material or a rod thereof. Lids can then be placed on either ends of the tube by welding, swaging or other mechanical process. The powder-in-tube or rod-in-tube are then mechanically deformed to form an elongated tape having biaxial texture in its sheath which was crystallographically a single crystal prior to deformation. The composite tube is then annealed to crystallize the superconductor precursor or other precursor so as to form epitaxially from the biaxially textured sheath of the tape. The sheath can be made of any suitable metal or alloy. In some cases the sheath is preferably made of either Ag or a Ag alloy. A superconductor article having enhanced biaxial texture can be formed by this method.